The present invention relates to an amplifier circuit capable of arbitrarily turning ON/OFF an input signal.
In the case of outputting a DTMF signal generated in a telephone to a line, a buffer amplifier circuit depicted in FIG. 2 has heretofore been used. The buffer amplifier circuit comprises: a differential amplifying unit D including a pair of N-channel type transistors 3 and 4 and a pair of P-channel type transistors 1 and 2 which are, as depicted in the figure, connected in a current-mirror form and provided as an active load on the side of a drain circuit of the transistors 3 and 4; and a P-channel type transistor 6 which is connected to a drain of the transistor 3 and constitutes an open drain type driver circuit. Based on this construction, the DTMF signal applied between the differential input terminals 8 and 9 of the differential amplifying unit D, which are connected to gates of the transistors 3 and 4, is fetched by an outside resistor 12 connected between an output terminal 11 of the driver circuit and a power supply -V. Note that a capacitor 7 is provided as an element for preventing oscillation.
Now, the output of the DTMF signal is required for a limitative short momentary period of time, i.e., during the dialing operation in a telephone system. The operation can be effected by supplying the differential amplifying unit D with a driving electric current only for such a desired period, and hence there is provided an N-channel type transistor 5 functioning as a switching element between the power supply -V and sources of the transistors 3 and 4. The arrangement adopted is such that a conductive state of the transistor 5 is controlled in accordance with a level of a control voltage signal C inputted from a control input terminal 10.
The control voltage signal C is outputted in association with the generation of the DTMF signal, and assumes earth or ground level (an "H" state) only when the transmission of the DTMF signal to the outside is needed. The differential amplifying unit D is caused to come into the operational state by making the transistor 5 conductive. On the other hand, if there is no necessity to output the DTMF signal, the control voltage signal C assumes the level of the power supply voltage -V (an "L" state). The transistor 5 is turned OFF, thus hindering a flow of electric current into the differential amplifying unit D. As a result, consumption of electric power is reduced.
In the conventional circuit described above in conjunction with FIG. 2, when turning OFF the transistor 5, the electric potential of the drain of the transistor 3 increases, thereby holding an OFF-state of the transistor 6 of the driver circuit. In the case of an active load type, however, the voltage of the drain of the transistor 3 is not increased up to earth or ground electric potential because of a threshold voltage of the transistor 2. Instead, a floating state is created, and it follows that the electric potential output of the open drain type driver circuit fluctuates. This causes a malfunction of a device or a circuit connected to the output of the buffer amplifier circuit.